The present invention relates to a plasma processing technology, in particular to an RF capacitively coupled plasma source with a broad range of operating gas pressure, specifically close to or more than atmospheric pressure.
Presently, the fields of technological application of plasma sources are growing rapidly, in particular in the manufacture of high tech devices for cleaning, etching, deposition, etc., in the semiconductor and micro-machine industries.
There exists many different types of plasma sources which assume different methods of plasma excitation in operating volumes of plasma devices. Within this variety, there is a separate group of capacitively coupled plasma sources in which the electromagnetic fields, with frequencies in the range of from a few MHz to tens of MHz (RF frequencies), are applied to plasma operating as one of the electrodes of an imaginary electric capacitor.
RF capacitively coupled plasma sources (hereinafter referred to as RF CCP sources) have wide application in plasma processing technologies. The present invention relates specifically to the RF CCP sources mentioned. Typical RF CCP sources of late generation are described in xe2x80x9cIndustrial Plasma Engineeringxe2x80x9d, Vol. 1, Principles, J. Reece Roth, Institute of Physics Publishing, 1995, p.p. 417-463. As shown in FIG. 12.12, page 443 of this book (see FIG. 1 of the present disclosure), a plasma source with outer ring electrodes 2 and 3 mounted outside of glass wall tube 1 serving as the working chamber, is one of the possible RF CCP sources. This arrangement of the electrodes enables one to avoid a plasma contamination with the electrode material. The operating gas required for such a process could be fed to one of the ends of the tube 1 under required pressure, whereby, at the supply of RF power to said electrodes 1 and 2 at a frequency of 1 to 100 MHz, plasma 4 generated inside the tube 1 is pressed by the electromagnetic field out of the inter-electrode space to contact with work-piece 5 installed at one of the tube ends (or at both ends of the tube 1).
However practical operation of the type of RF CCP source shown in FIG. 1 leads to significant radiation of the RF power from the source by the symmetrically placed electrodes 1 and 2 and especially by a combined electrode-plasma system forming a typical dipole antenna. The RF power applied to the electrodes 1 and 2 presses plasma from the inter-electrode space to both the ends of the tube 1. Therefore in the case where only one of the tube ends could be used for a plasma yield, a special device should be applied for cancellation of plasma propagation toward the second end of the tube. For example it could be a special dielectric labyrinth for plasma as it is applied in xe2x80x9cAiplasmaxe2x80x9d plasma sources (NAIS, Matsushita Electric Works, Ltd.).
For proper operation of the RF CCP source type described above, a voltage for igniting and a voltage for maintenance of the RF discharge plasma inside the tube 1 should be significantly less than the voltage necessary for electric breakdown between the electrodes outside the tube, including breakdown along the tube surface. In the process of operating the RF CCP source shown in FIG. 1, the significant quantity of heat should be evacuated from the electrodes 2, 3 and from the surface of tube 1 to have a proper electrode and tube temperature. To provide this heat evacuation, the electrodes 2, 3 should have a reliable mechanical contact with the surface of tube 1. The reliable contact between the electrodes and the tube surface is desirable also to avoid an RF microarcing between an outside surface of the tube and an inner surface of the electrodes at the area of their contact. However this assumes the electrodes 2, 3, mounted precisely on the surface of tube 1, are exposed in this way to electric surface-breakdown. To overcome this problem, the electrodes 2, 3, could be enveloped in a high-temperature dielectric and sealed from one another as is done in xe2x80x9cAiplasmaxe2x80x9d plasma sources (NAIS, Matsushita Electric Works, Ltd.).
A principal disadvantage of such a device is limitation in a gas pressure range and in a gas composition capable to provide a stable igniting and maintenance of plasma inside the tube with a reliable absence of electric sparks and breakdowns outside the tube 1. A second disadvantage of this device is a restriction in the active RF power absorbed in the source (300-400 Wt) due to obvious restriction in temperature and breakdown voltage for the insulators enveloping the electrodes. It is clear also, that breakdown voltage for any insulator is decreased with temperature increase, while the cost of such an insulator is increased drastically with an increase of an upper allowable temperature (note for example the material kinds PEEK, PBI and Polyimide). The third disadvantage is the problem of growing expenses for insulating electrodes having a large width, more than 25 cm, and corresponding to a rectangular cross-section of the discharge tubes required for linear plasma sources acceptable for industrial conveyor technologies.
It is an object of the present invention to provide the capacitively coupled RF plasma source of the external ring or collar type with the configuration of an RF loaded line type which is simple in design, inexpensive to manufacture, eliminates dissipation of power in an electromagnetic interference, provides effective transfer of energy from power supply to plasma, provides necessary electric insulation of the electrodes, provides necessary cooling of the electrodes with an operating tube surface, and provides RF power pressure on plasma only toward one of the ends of the operating tube for direct contact of plasma with an surface to be treated. Another object of the invention is to provide an efficient plasma generation at any chosen gas pressure and gas composition flowing in the operating tube, including aggressive mediums.
An RF loaded line type CCP source is formed as a terminating fragment of a loaded RF line (coaxial line or balanced stripline) including a plasma beam as a part of a line core. The core of a feeding coaxial cable is connected with a high voltage collar type electrode coupled capacitively with a plasma beam electrically continuing this central core, and coupled further capacitively with a collar type electrode extended to form a grounded outer coaxial shield enveloping both electrodes, and a plasma-beam is contained in an insulating tube passing through both collar type electrodes. The grounded shield of the source is connected with the outer braid of the feeding coaxial cable. A gaseous or vapor working medium required for a specific process is fed to the operating tube from the high voltage electrode end under a pressure of from several mTorr to several atmospheric pressures, depending on the process. The plasma beam yields or is discharged from the operating tube at the grounded end under action of the gas flow and an RF field pressure. The power supply operates at a frequency of from 10 MHz to 100 MHz or higher and has a power rating of from tens of W to several kW. To provide a stable plasma excitation at any chosen gas or vapor pressure and composition in the operating tube, an interior cavity is formed between the ring or collar type electrodes with the operating tube and the inner surface of the outer enveloping shield and is filled with a gas or a special gas mixture having a required pressure or with an oil to provide a required electrode insulation against an RF breakdown outside the operating tube, i.e. in the source interior cavity. To form a linear plasma beam required for a large surface treatment, the RF loaded line type CCP source of the coaxial configuration is flattened in a plane passing through the source axis to the geometry of the conventional RF stripline.